Interfacial Corrections of Elastomers for Kubelka-Munk Theory Using Non-contact Measurements

Reflectance measurements using a non-contact system of 45° illumination and 0° (normal) observation provide more accurate data for translucent natural or restorative materials, yet the interfacial reflection corrections (IRCs) have not been evaluated for this system. Objective: The specific objective of this study is to determine which of 3 IRC methods has the least error when using Kubelka-Munk Theory (K-MT) to model the spectral reflectance of maxillofacial elastomer pigmented to produce natural skin colors. Method: Samples at varying thicknesses of each of 19 shades of skin-colored maxillofacial elastomer were measured using a non-contact 45°/0° spectral reflectance system on each of black, gray and white backings. Reflectance values at each wavelength from 380nm to 780nm in steps of 2nm for each shade were fit by non-linear regression to K-MT using no IRC and using IRCs appropriate for a 45°/0° geometry using two different values of the internal reflection coefficient, one for opaque (Op) and one for completely scattering translucent (Tr) materials. The errors associated with each model at each wavelength were analyzed using repeated-measures ANOVA. Results: The errors were found to be statistically significantly different for the interaction between the model and the wavelength (P<0.001), with the average Error Mean Square over all wavelengths of .00038 for K-MT with no IRC, 0.00023 with Op IRC, and 0.00015 with Tr IRC. Conclusion: Although further research is indicated for determining if a custom IRC is even more appropriate for application to maxillofacial materials pigmented to appear like human skin, minimal error can be obtained using K-MT with the Tr IRC.

This study was supported by Student Research Program of The Ohio State University College of Dentistry.